Complex Haptics Deliver a Pinch, a Stretch, or a Tap

Most haptic interfaces today are limited to simple vibrations. While visual displays and audio systems have continued to progress, those using our sense of touch have largely stagnated. Now, researchers have developed a haptics system that creates more complex tactile feedback. Beyond just buzzing, the device simulates sensations like pinching, stretching, and tapping for a more realistic experience.

The sensation of touch is the most personal connection that you can have with another individual," says John Rogers, a professor at Northwestern University in Evanston, Ill., who led the project. It's really important, but it's much more difficult than audio or video."

Co-led by Rogers and Yonggang Huang, also a professor at Northwestern, the work is largely geared toward medical applications. But the technology could be used in a wide range of uses, including virtual or augmented reality and the ability to feel the texture of clothing fabric or other items while shopping online. The research was published in the journal Science on 27 March.

A Nuanced Sense of Touch

Today's haptic interfaces mostly rely on vibrating actuators, which are fairly simple to construct. It's a great place to start," says Rogers. But going beyond vibration could help add the vibrancy of real-world interactions to the technology, he adds.

These types of interactions require more-sophisticated mechanical forces, which include a combination of both normal forces directed perpendicular to the skin's surface and shear forces directed parallel to the skin. Whether through vibration or applying pressure, forces directed vertically into the skin have been the main focus of haptic designs, according to Rogers. But these don't fully engage the many receptors embedded in our skin.

The researchers aimed to build an actuator that offers full freedom of motion, which they achieved with very old physics," Rogers says-namely, electromagnetism. The basic design of the device consists of three nested copper coils and a small magnet. Running current through the coils generates a magnetic field that then moves the magnet, which delivers force to the skin.

What we've put together is an engineering embodiment [of the physics] that provides a very compact force delivery system and offers full programmability in direction, amplitude, and temporal characteristics," says Rogers. For a more elaborate setup, the researchers also developed a version that uses a collection of four magnets with different orientations of north and south poles. This creates even more complex sensations of pinching, stretching, and twisting.

Haptics at Your Fingertips-or Anywhere

Because fingertips are highly sensitive, only small forces are needed for this application. John A. Rogers/Northwestern University

Although much of the previous work in haptics has focused on fingertips and the hands, these devices could be placed elsewhere on the body, including the back, chest, or arms. However, these applications may have different requirements. Compared with places like the back, the fingertips are highly sensitive-both in terms of the force needed and the spatial density of receptors.

The fingertips are probably the most challenging in terms of density, but they're easiest in terms of the forces that you need to deliver," says Rogers. In other use cases, delivering enough power may be a challenge, he acknowledges.

The force possible may also be limited by the size of the coils, says Gregory Gerling, a systems engineering professor at the University of Virginia and former chair of the IEEE Technical Committee on Haptics. The coil size dictates how much force you can generate, and at a certain point, the device won't be wearable. However, he believes it is sufficient for VR applications.

Gerling, an IEEE senior member, finds the use of magnetism in multiple directions interesting. Compared with other approaches that are based on hydraulics or air pressure, this system doesn't require pumping fluids or gases. You can be kind of untethered," Gerling says. Overall, it's a very interesting, novel device, and maybe it takes the field in a slightly new direction."

Applications in VR, Neuropathy, and More

The clearest application of the device is probably in virtual or augmented reality, says Rogers. These environments now have highly sophisticated audio and video inputs, but the tactile component of that experience is still a work in progress," he says.

Their lab, however, is primarily focused on medical applications, including sensory substitution for patients who have lost sensation in a part of the body. A complex haptics interface could reproduce the sensation in another part of the body.

For example, nerve damage in people with diabetic neuropathy makes it difficult for them to walk without looking at their feet. The lab is experimenting with placing an array of pressure sensors into the base of these patients' shoes, then reproducing the pattern of pressure using a haptic array mounted on their upper thighs, where they still have sensation. The researchers are working with a rehabilitation facility in Chicago to test the approach, mainly with this population.

Continuing to develop these medical applications will be a focus moving forward, says Rogers. In terms of engineering, he would like to further miniaturize the actuators to make dense arrays possible in regions of the body like the fingertips.

Feeling the Music

Additionally, the researchers explored the possibility of using the device to increase engagement in musical performances. Apart from perhaps feeling vibrations of the bass line, performances usually rely on sight and sound. Adding a tactile element could make for a more immersive experience, or help people with hearing impairment engage with the music.

With the current tech, basic vibrating actuators can change the frequency of vibration to match the notes being played. While this can convey a simple melody, it lacks the richness of different instruments and musical components.

The researchers' full-freedom-of-motion actuator can convey a more vibrant sound. Voice, guitar, and drums, for instance, can each be converted into a delivery mechanism for a particular force. Like with vibration alone, the frequency of each force can be modulated to match the music. The experiment was exploratory, Rogers says, but it exploits the advanced capabilities of the system.